Tactics
Hassan appears only a few times in the story. Normally, he moves stealthily, collecting intelligence per the orders of his Master, Matou Zouken. Hassan only shows himself when it is time to kill an enemy.
Hassan himself not a strong Servant. He displays his combat strength through surprise attacks, and is ill-suited to frontal assaults. Thus, Hassan waits for opportunities to arise with an invariable/absolute/certain chance of killing the target. There is no need for him to pursue the superhuman Servants. Instead, he waits for the perfect chance to get close to a Master, whom he can easily defeat...

Fate/complete material III; Servants of the Fifth Holy Grail War, pg. 32-33